b 1. Field of the Invention
The present invention relates to a process for surface modifying support membranes and to new and improved membranes produced from such process. More particularly, the present invention relates to a process for preparing a surface-modified support membrane wherein an activated modifying polymer is irreversibly adsorbed onto essentially all of the surface area of the membrane. The coated membrane is subsequently treated under conditions which effect substantially uniform cross-linking of the modifying polymer by way of the activated portions. In a particularly preferred embodiment, a uniform layer of a chlorohydrin-functionalized hydroxypropylcellulose material is irreversibly adsorbed onto essentially all of the surface area of a porous hollow fiber membrane. The chlorohydrin moiety is then converted to the corresponding epoxide which subsequently reacts with the hydroxyl groups of adjacent HPC molecules in order to cross-link the monolayer of HPC. The resulting coating membrane is resistant to cleaning agents such as surfactants, solvents, and caustics.
2. Prior Art
Ultrafiltration and microfiltration membranes utilized in industry, particularly in the food processing industry, and in environmental applications are typically hydrophobic membranes which may be surface-modified with a hydrophilic material to reduce fouling. Such membranes are known to foul after a certain period of use. When such membranes are utilized in a manufacturing facility, attempts are made to clean the membranes rather than replace them. Preferred cleaning methods include washing with sodium hydroxide, with surfactants, and/or with aqueous ethanol. Although some coatings are stable to the sodium hydroxide wash, washing with ethanol, which is a preferred cleaning method in the food processing industry, removes the hydrophilic coating. Thus, there is a need for membranes which are stable to ethanol.
U.S. Pat. No. 4,794,002 discloses a process for irreversibly adsorbing a modifying polymer onto a support surface. This process involves soaking the support membrane in a solution of the modifying polymer dissolved in an appropriate solvent for a period of time sufficient for the modifying polymer to adsorb onto the support membrane. The resulting modified membranes are then washed with excess solvent. This particular process is suitable for surface modifying a variety of polymeric support surfaces by irreversibly adsorbing thereon a suitable modifying polymer which possesses the desired chemical characteristics. For example, a fabric made of a polymeric material such as nylon can be surface modified to increase its ability to wet and thereby render clothes made of such a modified fabric more comfortable. Employing this process, one can surface modify and render substantially hydrophilic various laboratory and medical devices which often are made of hydrophobic polymeric materials which interact undesirably with protein material. The surface modifications can be further stabilized by cross-linking or can be derivatized in order to incorporate desired reactive groups, such as, for example, ion exchange groups, e.g., derivatized using chloroacetic acid. However, the cross-linking is accomplished by depositing a cross-linking agent onto a deposited layer of polymeric material. For example, ethylene glycol diglycidyl ether, or epichlorohydrin, in 1N NaOH is deposited onto a layer of modifying material. In order to deposit the cross-linking agent, however, a solvent evaporation process is utilized. This results in a nonuniform distribution of the agent and loss of the agent during the evaporation process. Furthermore, the degree of cross-linking is dependent upon, among other things, the solubility of the cross-linking agent and the boiling point thereof in relation to that of the solvent.
U.S. Pat. No. 4,618,533 discloses surface modified membranes having passivation coatings which membranes are prepared by wetting a polymeric porous membrane and coating directly onto the entire surface thereof a monomer grafted and cross-linked in situ onto the membrane. Such membranes are prepared utilizing high concentrations of a free radical initiator and heating the monomer-containing membrane for a short period of time, such as from 2 to 30 minutes, at a temperature between 60.degree. and 95.degree. C. Polymerization in this manner is extremely difficult to control and membranes resulting from such process do not have a uniform coating. Furthermore, the flow characteristics of such membranes are significantly reduced.
Cross-linking of hydroxypropylcellulose is typically accomplished utilizing toluene diisocyanate (TDI) or dimethylolurea (DMU). However, the cross-links which result are hydrolytically unstable and therefore continuously bleed hydrolysis products into the process streams. TDI results in the release of 1,3-diaminobenzene and DMU results in the release of formaldehyde. Obviously, neither of these hydrolysis products would be toxicologically desirable in either a food processing or a biological application.